Salts of high molecular weight sulfonic acids of organic compounds have found use as rust inhibitors in motor fuels and lubricating oils, and as rubber plasticizers. See, for example, R. G. King and G. W. Thielcke, U.S. Pat. No. 2,764,548, which is incorporated herein by reference. Other uses for such salts are found in textile treating solutions, and as wetting agents.
The most important salts commercially are derived from mono-, di- and trisulfonic acids of aliphatic and aromatic hydrocarbons, containing from about 6 to about 60 carbon atoms, including branched and straight chain alkanes, mono- and poly- cyclic aromatics and alkyl substituted such compounds. The molecular weights will range from about 150 to about 1500. Monosulfonic acids with molecular weights greater than about 350 tend to be oil soluble while those with lower molecular weights tend to be water soluble. In the case of di- and tri-sulfonic acids, the minimum molecular weights for oil solubility tend to be higher. Particularly valuable salts are alkali metal, alkaline earth metal, lead and zinc salts of such organosulfonic acids as dinonylnaphthalene mono- and disulfonic acid. Special mention is made of such salts, and especially the sodium, potassium, lithium, calcium, magnesium, barium and zinc salts of dinonylnaphthalene disulfonic acid. The latter family of salts are disclosed in the said patent, U.S. Pat No. 2,764,548. The barium, calcium and lithium salts, particularly, form products having exceptional rust inhibiting properties.
Commercially, such salts are often prepared by using oxides or hydroxides of the corresponding metal to neutralize the sulfonic acid. However, this is disadvantageous because the oxides and hydroxides are highly caustic and, in some cases, toxic. Moreover, end-point control is difficult and requires accurately stopping the flow of neutralizing agent. Exact neutrality can be very important because, for example, overneutralized metal salt sulfonates tend to be difficult to filter. Also, the salts of sulfonic acids are frequently used in combination with ester lubricants or other additives including amines and weak acids which cannot tolerate free acidity or basicity.
In the said patent, U.S. Pat. No. 2,764,548, it is suggested that the metal be reacted in the form of a carbonate with the organosulfonic acid, and this indeed is more economical, much safer, and more controllable. However, even with the carbonate, as is specifically taught in the patent, end-point control is very important from a processing standpoint, because if an excess of carbonate is added, filtration is necessary to free the finished product from turbidity. Thus, it is taught that the batch should be transferred to solvent recovery when the neutrality point has been reached.
Mackinnon, U.S. Pat. No. 2,702,280, describes the preparation of sulfonic acid salt detergents by the neutralization of the corresponding benzenesulfonic acid with carbonates, then with an alkali metal hydroxide in two steps. It is said to be essential to carry out the reaction only to 70 to 85% completion with carbonate and then to go the rest of the way with the hydroxide, 15 to 30% of the neutralization being achieved with the hydroxide -- a very substantial amount.
Jacob et al, U.S. Pat. No. 2,688,035, also deal with neutralizing sulfonic acid with alkali carbonates, but use a small excess of the latter, and leave it in the reaction product -- to prevent formation of free acid. Jacob et al furthermore, before making any adjustments with acid, homogenize the mixture containing the excess carbonate and measure the pH on a sample in which carbonate is homogeneously dispersed therethrough.
It has now been discovered that, in such systems, carbonate neutralization appears to be limited by a carbonate (bicarbonate)/acid equilibrium, which prevents complete neutralization of the sulfonic acid present. As will be understod by those skilled in this art, the term "carbonate/acid equilibrium," as used herein, means also "bicarbonate/acid equilibrium," because the ultimate working equilibrium includes the bicarbonate ion. This discovery was not foreshadowed by the prior art because King et al call for neutralization precisely with carbonate only; Mackinnon doesn't use enough carbonate to reach any equilibrium point; and Jacob et al use a system with excess carbonate and measure the pH on carbonate-containing samples -- which obscures the presence of an equilibrium. In applicants' work, measurements on carbonate-free samples have determined that the equilibrium point is just shy of a true end point, regardless of the excess of carbonate present. As a result of this unexpected observation, which could not have been made by King et al, Mackinnon, or Jacob et al, the subsequent finding that the addition of a very tiny amount of strong base pushes the neutralization to completion, and gives a substantial improvement in process economics and in the quality of the product, is most surprising. These process advantages have wide applicability to the formation of numerous metal salts of organosulfonic acids. Surprisingly, it is critical to add the strong base after the carbonate. If the order is reversed, all advantages are lost. Moreover, if water is excluded from the process, no neutralization occurs with metal carbonates. The key requirement in selection of the strong base is to use one which has a base strength sufficiently greater than that of the bicarbonate ion to effect complete neutralization. The pKa of the conjugate acid (H.sub.2 CO.sub.3) of the bicarbonate ion (HCO.sub.3 -) is 6.38. Accordingly, suitable strong bases will be those whose conjugate acid have pKa's of greater than 7.
In the systems of the present invention, the relevant equilibrium would be given by CO.sub.2 +H.sub.2 O.revreaction. H.sup.+ +HCO.sub.3 -, and the equilibrium constant is: ##EQU1## Taking the expression further, ##EQU2##
Because the known value of pK for this equilibrium is 6.38, it follows that for (HCO.sub.3 -)=(CO.sub.2), the equilibrium pH will equal the pK or 6.38. Thus, the relative concentrations of HCO.sub.3 - and CO.sub.2 are important. since the solubilities of bicarbonates tend to be greater in acid media while that of CO.sub.2 is greater in basic media, the pH at equilibrium is very close to pK.